1. Field of the Invention
The present invention relates to digital data transmitting apparatus and receiving apparatus that can be applied to systems in which digital apparatuses or equipment, such as a DVD (digital video disc) recording/reproduction apparatus, digital VCR, and digital TV, are connected to each other via suitable interfaces having a fixed data transfer rate, such as an IEEE 1394 bus.
2. Prior Art
In a conventional digital data transmission system, digital recording/reproduction apparatuses, such as an optical disc (e.g., DVD) reproduction apparatus, digital TV, digital VCR, and others, are connected to each other via an IEEE-1394 bus that serves as an intelligent interface, so that data contents, such as pictures and music, are transmitted and received between these digital apparatuses. When digital data is transmitted and received between these apparatuses, the system checks if each of the apparatuses operates in a manner as intended by the creator or producer who created the data contents, and, if any one of the apparatuses does not operate as intended by the creator, data transfer is prohibited, to thereby prevent the user from illegally copying the picture and/or music contents against the intention of the creator. It is thus possible to construct a digital data transmission system that satisfies the demands of both suppliers of the contents and general users by permitting digital copying under certain restrictions, utilizing the mutual authentication function as described above.
The IEEE-1394 interface, when it is designed to handle 1000 Mbps, for example, is able to transmit 4915 units of data each counted as a unit of transmission of certain data, during a basic transmission cycle of 125 μS. The number of units that are transmittable during the basic cycle corresponds to a bit rate of each type of signal to be transmitted. For example, 1200 units is employed when transmitting picture information at the average bit rate of 10 Mbps, and 704 units is employed when transmitting music information at the average bit rate of 2 Mbps. In the transmitter-side apparatus, therefore, the transmission rate is determined in advance based on the contents to be transmitted, and then transmission is carried out at the transmission rate thus determined.
On the other hand, picture/music information that is read out from DVD is read out at a variable bit rate since the information was encoded at a variable bit rate according to the MPEG (Moving Picture Expert Group) method. Namely, the frequency of reading picture information containing more active or fast-moving pictures differs from that of reading picture information containing less active or slow-moving pictures. In this case, the read-out timing is determined by monitoring the storage state of data in an input buffer memory, for example. A conventional data transmission mode or method for transmitting the information read at such a variable rate through the fixed-rate IEEE 1394 bus is described at Part 4 of IEC 61883.
The IEC 61883, however, only specifies the manner of transmitting data that were packeted as an MPEG-TS, onto the IEEE-1394 bus, and provides no description on how to convert various information recorded on a DVD into the form of an MPEG-TS signal, nor specifies an effective transmission method. Also, in the data transmission method of IEC 61883, such information that is required to inform the receiver of the reproduction state of the transmitter-side apparatus is not transmitted to the receiver-side apparatus, causing a problem that the receiver-side apparatus fails to perform the same reproducing operations as performed by the transmitter-side apparatus. Namely, serial packets read at a variable rate from DVD contain time stamps representing the reproduction timing, and display timing of each image or picture is determined through an operation to compare the time stamps with the absolute time or pulses produced by the reproduction apparatus. If the reproduction apparatus receives a command to perform fast-forward, reverse reproduction or other operations, the time stamps of picture information may be skipped at certain intervals or advanced in the reverse direction. This will cause no problem when the reproduction apparatus itself performs such an operation according to a command entered through the reproduction apparatus, but, when data is transmitted as it is from the transmitter-side apparatus to the receiver-side apparatus, such information that instructs a reproduction manner is not supplied to the receiver-side apparatus, thus causing a problem in reproducing operations of the receiver-side apparatus.
Also, since digital data is transmitted from the transmitter-side apparatus in accordance with the state of buffering of the reproduction apparatus in the transmitter-side apparatus, the data may be transmitted even if a buffer(s) of the reproduction apparatus in the receiver-side apparatus is/are in the full state, or may not be transmitted even if the buffer(s) is/are in the empty state.